Chick not eating/drinking and looks weak

We received these chicks from the post office today and they all seemed fine (assumed 2 days old?). My parents decided to give them some expired baby parrot feed as substitute while we went out to buy food for them; bought some small seeds and grain for parakeets as temporary food while the chick feeds comes in the mail. All the chicks seemed like they really like the seeds, except one who was sleeping in the corner and refused to move. It refused to eat or drink, or even stand up while I'm holding it. It looks extremely weak or tired, and I have tried feeding it sugar water. But after that it has vomited clear mucus twice and now looks even weaker than earlier, with open mouth breathing. I don't think the cause of that could be overheating since I have temporarily separated it from others (and the heat lamp) to keep a close eye on. There are no signs of pasting. What can I do for her right now?

Sorry, i don't have any suggestions for your weak chick, but you may wish to blitz your existing seeds in a food processor to a "grainy powder" size as in the absence of grit in their crops, you chicks will find it difficult to digest seeds.

It needs the heat though. Chicks need to be kept at a temp of about 99F and you gradually reduce the heat thru weeks. Sav a chick (electrolyte) has worked for me in the past. I agree with CTKen. Grind up those seeds, or get some chick starter from the feed store. Their little bodies can't handle all those types of food yet.